This invention relates to a bias-magnetic-field generating method and apparatus for magneto-optical memory system which records, reproduces and erases information on a recording medium made of a magneto-optical recording material.
It has previously been known to provide a magneto-optical memory system which records, reproduces and erases information on a magneto-optical information storing medium such as a magneto-optical disc by way of the magneto-optical effect.
In the memory system of this type, points on a magneto-optical disc may be heated by laser beam irradiations in the presence of a bias-magnetic-field, and information is recorded or erased according to a altering direction of the bias-magnetic-field applied to the disc.
One of the magneto-optical memory system in the prior art is shown in FIG. 1. A magneto-optical disc 1 is placed on a turn table mounted on a spindle motor 2. A laser diode 3 which is connected to a drive circuit 4 emits a laser beam. The laser beam irradiates a point of a magnetic film on the disc 1 under the focus control by means of an objective lens 5. Apart from the disc 1, a coil 6 is placed on the opposite side of the beam-irradiated face of the disc 1. The coil 6 is connected to a coil drive circuit 7 via a switch 8, and generates the bias-magnetic-field.
In this system, information is recorded on the rotating disc 1 in the following procedure hereinafter decribed.
In the initial state, no information is recorded on the disc 1 and the magnetic film on the disc 1 is uniformly magnetized in one direction, for instance, upward with respect to the disc 1. The laser diode 3 emits the laser beam in the pulsive ON-and-OFF mode by means of the drive circuit 4 depending on the information to be recorded on the disc 1. The magnetic film, which is irradiated by the focused laser beam, is heated at a spot where the magnetic phase of the film is changed from the initial ferromagnetic phase to the para-magnetic phase when the temperature exceeds a curie temperature. Namely, the magnetic film on the disc 1 loses its initial spontaneous magnetization at the spot. On the other hand, since the disc 1 is rotating, the heated spot is moved away from the beam-irradiating position, and the spot is cooled and recovers its ferromagnetic phase in the presence of the bias magnetic-field. While the bias-magnetic-field is generated by the coil 6 in the direction selected by the switch 8, e.g. downward relative to the disc 1, the ferromagnetic phase in the spot is oriented downward. Thus the information of " 1" or "0" is recorded onto the disc 1 at the spontaneous magnetized spot, called "pit", depending on the direction of the applied bias-magnetic-field.
In the case of the information erasing, following procedure similar to the recording process is performed as hereinafter described.
The laser diode 3, which is driven by the drive circuit 4, emits laser beam and irradiates the rotating disc 1. Bias-magnetic-field, a direction of which is selected by the switch 8, for instance, upward relative to the disc 1, is also applied to the disc 1. Thus the magnetic film storing the recorded information is heated by the laser beam irradiation and then cooled in the presence of said bias-magnetic-field. Therefore, the film loses its spontaneous magnetization of the recorded "pit" and obtains upwardly oriented spontaneous magnetization same as the initial state. Consequently, the recorded information on the magnetic film of the disc 1 is erased by means of the laser beam and the accompanied bias-magnetic-field.
However, the magneto-optical memory system in the prior art has its own problems as follows. In the prior art system, the coil 6 wastes a large amount of electric power, for example more than 1 watt, to produce enough bias-magnetic-field strength for the information recording or erasing. This energy dissipation also raises the coil temperature and the electric resistance of the coil winding is increased. The increase in the winding resistance becomes remarkable when the coil is driven in long-time-lasting operation. Consequently, this increase in coil resistance reduces a current flowing through the coil so that the bias-magnetic-field strength becomes decreased. Therefore, inappropriate information recording or erasing is caused by the insufficient bias-magnetic-field strength.
To solve the above described shortcoming, a permanent magnet is proposed to replace the coil. However, this causes another problem in the memory system. Namely, a complicated mechanism must be installed for altering the direction of the bias-magnetic-field for example.
On the other hand, the switch 8 for altering the direction of the bias-magnetic-field in the prior art causes another problem as follows. Generally, the switch 8 comprises a field polarity selecting circuit with four transistors in which two transistors work as a current source driver and other two transistors work as a current sink source driver for altering the coil current. When a command signal, e.g. write command or erase command signal, is supplied to the polarity selecting circuit, two transistors (one for the current source and another for the current sink) are selected and suitable bias-magnetic-field is applied to the disc by means of the energized coil. Consequently, a complicated polarity selecting circuit with four transistors must be installed in the memory system in the prior art.